PSA processes (Pressure Swing Adsorption) are being used more and more for producing oxygen with limited purity from atmospheric air. There is, of course, a constant preoccupation with reducing the cost of the oxygen to be produced. In order to do this, use may be made of adsorbents which are particularly specific and effective for O.sub.2 /N.sub.2 separation, such as lithium-exchanged zeolites. However, the cost of these adsorbents is very much greater than that of conventional adsorbents such as 5A or 13X molecular sieves, so that one condition for their acceptance is that a small amount of adsorbents is necessary.
In order to minimize the volumes of adsorbents to be utilized, it has been proposed to shorten the cycle, in particular overlapping steps which have normally been sequential. Thus, the air may be introduced into the adsorber as soon as possible after the end of the purge/elution step, or immediately following it. However, all other things being equal, this introduction of air at low pressure has a detrimental effect on the propagation of the adsorption fronts, which advance commensurately faster if the pressure in the adsorbent is low.
This is true both for the retention of nitrogen and for that of atmospheric impurities (water, CO.sub.2, traces of hydrocarbons, etc.), for which use is made either of a specific bed such as an alumina or silica gel bed, or an adsorbent used for O.sub.2 /N.sub.2 separation. In all cases, the introduction of air at low pressure actually requires greater volumes of adsorbents, which is contrary to the intended result.
Furthermore, enlarging the zone allocated to retaining the impurities increases the volume of air which is unproductively compressed then pumped, with a negative effect on the specific energy.
One way of limiting these drawbacks is to minimize the quantity of air introduced at the lowest pressures by arranging for the pressure in the adsorber to rise very rapidly.
Thus, according to the teaching of EP 758 625 A1, FIGS. 4 and 5, after the purge/elution step the following steps are employed in a cycle of the aforementioned type, the duration of which is 120 seconds:
This leads to sequencing, in spite of the relatively long duration of the cycle, steps which have very short durations--from one second to a few seconds at most--with multiple valve-opening/closing operations in the short corresponding lapse of time.